1. Field of the Invention
The present invention relates generally to an active filter circuit and, more particularly, to a filter circuit suitable for a filter incorporated into an IC.
2. Related Art Statement
A cut-off frequency and a Q factor at this frequency are main factors for determining I/O characteristics of a filter circuit. A secondary or higher filter causes a difficulty of adjustment thereof. This point at issue will be clarified in detail referring to an illustrated circuit.
FIG. 5 shows a conventional example in which a secondary low-pass filter circuit is constructed of two pieces of conductance amplifiers.
Provided are transconductance amplifiers 501, 502, wherein conductances are proportional control currents I1, I2, for outputting currents proportional to input voltages. A non-inverting input terminal of the amplifier 501 is connected to an input terminal IN of the filter circuit. An inverting input terminal thereof is connected to an output terminal OUT of the filter circuit. The output terminal is connected to the other terminal of a capacitor cl one terminal of which is grounded. A non-inverting input terminal of the amplifier 502 is connected to an output terminal of the amplifier 501. An inverting input terminal thereof is connected to the output terminal OUT of the filter circuit. The output terminal is connected to the other terminal of a capacitor c2 one terminal of which is grounded.
Further, an input terminal of a buffer 503 with a gain of "1" is connected to the output terminal of the amplifier 501. An output terminal thereof is connected to a (+) input terminal of the amplifier 502. An input terminal of a buffer 504 with a gain of "1" is connected also to the output terminal of the amplifier 502. An output terminal thereof is connected to the output terminal OUT of the filter circuit.
With this configuration, the secondary low-pass filter is constructed of: an integrating element formed of a conductance of the amplifier 501 and a capacitance of the capacitor c1; and an integrating element formed of a conductance of the amplifier 502 and a capacitance of the capacitor c2.
A current mirror circuit 506 duplicates a current I0 outputted from a constant-current source 505 to two systems. One output terminal of this current mirror circuit 506 is connected to a control input terminal of the amplifier 501. a constant-current source 507 for outputting a current IQ is connected to a connecting point therebetween. A current I1 (=I0-IQ) is supplied as a control current to the amplifier 501. The outer output terminal of the current mirror circuit 506 is connected to the control input terminal of the amplifier 502. A current I2 (=I0) is inputted as a control current to this amplifier 502.
A transfer function of a filter main part consisting of the amplifiers 501, 502, the buffers 503, 504 and the capacitors c1, c2 is, as widely known, given by the following formula: ##EQU1## where Vin is the voltage at the input terminal IN, Vout is the voltage at the output terminal OUT, gm1 is the conductance of the amplifier 501, gm2 is the conductance of the amplifier 502, C1 is the capacitance of the capacitor c1, and C2 is the capacitance of the capacitor c2.
On the other hand, the following is a general equation of the transfer function of the secondary low-pass filter: EQU T(s) =.omega.0.sup.2 /(s.sup.2 +(.omega.0/Q)s+.omega.0.sup.2)(2)
where .omega.0 is the cut-off angular frequency, this angular frequency .omega.0 being given such as .omega.0=2.pi. fc when letting fc be the cut-off frequency, and Q is the gain (dB) of the filter circuit at the cut-off frequency fc.
In the case of the secondary low-pass filter shown in FIG. 5, .omega.0, Q are expressed respectively by the following formulae: EQU .omega.0=(gm1 gm2/C1 C2).sup.1/2 ( 3) EQU Q=((gm1/gm2) (C2/C1)).sup.1/2 ( 4)
By the way, the conductances gm1, gm2 are proportional to the control currents I1, I2, and let the proportional constants be replaced by K1, K2. EQU gm1=K1 I1=K1 (I0+IQ) EQU gm2=K2 I2=K2 I0 (5)
(where IQ is the output current of the constant-current source 505, this output current being allowed to take both positive and negative values. )
The formulae (3) and (4) are rewritten by the relationship shown in this formula (5) as follows: ##EQU2##
It can be known from this relationship that .omega.0 is proportional to ((I0+IQ) I0).sup.1/2, and Q is proportional to (1+IQ/I0).sup.1/2.
Now, there is given a consideration of how .omega.0 and Q are adjusted. .omega.0 can be efficiently adjusted by varying I0, and Q can be also efficiently adjusted by varying IQ.
When I0 is changed to adjust .omega.0, however, it follows that Q is also varied. Besides, IQ is changed to vary Q, and .omega.0 is simultaneously varied. Consequently, when adjusting Q and fc of the low-pass filter to desired values, any one of fc and Q is at first shifted, and the other is next shifted. The problem is such that these operations have to be repeated.
This problem is not inherent limitedly in the low-pass filter but may be, it can be said, applied to a variety of current control active filters that constitute high-pass filters, band-pass filters, notch filters, etc.
As stated above, the conventional active filter circuit is incapable of independently adjusting the cut-off frequency and the gain at this frequency point. The adjustment thereof is time-consuming.